Color consistency is of major importance in commercial printing, and much effort is put into producing and preserving the exact colors defined by the customer for a print job.
Digital halftone color printing systems use a small number of differently colored marking agents—a typical printing system may, for example, use four inks (cyan, yellow, magenta and black). For each ink, a respective Look Up Table (LUT) is provided to map a requested ink coverage, derived from input image data, to the digital screen that should be used on the print according to the current printed state.
In order to enable accurate color reproduction, the LUT for each ink is calibrated by a process that involves using only that ink to print, for example, fifteen patches each using a respective one of the following percentage digital screens: 2, 4, 6, 9, 12, 14, 16, 18, 23, 27, 33, 40, 50, 65 and 80, and then measuring the resulting actual ink coverage printed in respect of each screen. This enables a screen-size/coverage mapping to be generated and the thick line 10 in FIG. 1 of the accompanying drawings illustrates a typical mapping curve 10 derived in this way. The reasons for the non-linearity of the curve 10 include ink and printing-system properties such as dot gain and dot loss. The mapping generated for each ink is stored in the corresponding LUT.
Following the calibration process, whenever it is desired to print a particular coverage t of one of the inks, the corresponding LUT is used to map the requested coverage to the digital screen r that needs to be sent to print in order to have coverage t printed.
However, with time, the printing system state starts to change (drift) throwing out the mappings held by the LUT—that is, printing with a specific digital screen results in different coverage from the one indicated by the LUT mapping. This gives rise to prints that are inconsistent with earlier prints of the same input image data and this can be particularly significant over long print runs. In order to preserve consistency, the print run has to be interrupted and the LUTs re-calibrated. Since printing the calibration job requires time and additional 4-13 pages, print costs are increased; in addition, print workflow is interrupted.
In order to save time and consumables, shorter color calibration processes have been introduced based on the approximation that the color drifts in the printing system are small and that the required LUT correction can be modelled by some function. These approximations enable the LUTs to be updated to take account of drift based on measurements of only a few single-ink patches for each ink.
A number of proposals have previously been made to calibrate (including re-calibrate) the LUTs by making measurements of the customer job being printed; one such proposal is described in WO-A-2008/030522 (Hewlett-Packard Development Company). There are a number of difficulties involved in using the customer print job for LUT calibration, notable among which is that there will generally be few, if any, areas printed with only a single ink.